Relic proto-stellar discs and the origin of luminous circumstellar interaction in core-collapse supernovae

A small fraction of core-collapse supernovae (SNe) show evidence that the outgoing blast wave has encountered a substantial mass ∼1–10 M⊙ of circumstellar matter (CSM) at radii ∼102–103 au, much more than can nominally be explained by pre-explosion stellar winds. In extreme cases, this interaction may power the most luminous, optically energetic SNe yet discovered. Interpretations for the origin of the CSM have thus far centred on explosive eruptions from the star just ∼ years−decades prior to the core collapse. Here, we consider an alternative possibility that the inferred CSM is a relic disc left over from stellar birth. We investigate this hypothesis by calculating the evolution of proto-stellar discs around massive stars following their early embedded phase using a self-similar accretion model. We identify a brief initial gravitationally unstable (‘gravito-turbulent’) phase, followed by a much longer period of irradiation-supported accretion during which less effective non-gravitational forms of angular momentum transport dominate. Although external influences, such as the presence of a wide binary companion, may preclude disc survival in many systems, we find that massive (∼1–10 M⊙) discs can preferentially survive around the most massive stars. Reasons for this perhaps counterintuitive result include (1) the shorter stellar lifetimes and (2) large photoevaporation radii (∼103 au) of very massive stars; (3) suppression of the magnetorotational instability due to the shielding from external sources of ionization and (4) relative invulnerability of massive discs to lower mass stellar collisions and luminous blue variable eruptions. Furthermore, discs with radii ∼102–103 au are picked out by the physics of the embedded stage of accretion and the requisite conditions for subsequent disc survival. The disc mass, radius and scaleheight at core collapse typically result in an ∼10 per cent efficiency for converting the kinetic energy of the exploding star into radiation, potentially producing a total electromagnetic output of ∼1050–1051 erg. We identify two regimes of disc–supernova interaction, which are distinguished by whether the shocked disc radiates its thermal energy before being engulfed by the expanding SN ejecta. This dichotomy may explain the difference between very luminous supernova which show narrow H line emission and those which show no direct evidence for hydrogen-rich CSM interaction. Because very luminous SNe are rare, testing the relic disc model requires constraining the presence of long-lived discs around a small fraction of very massive stars.